This invention relates to a composition of matter having a positive thermal coefficient of resistance and, more particularly, it is concerned with such materials suitable for electrochemical cells having internal temperature protection.
A particularly effective class of primary electrochemical cells employs liquid cathode material as a component of an electrolyte solution. Among the liquid cathode materials found useful are fluid oxyhalides, fluid non-metallic oxides, fluid non-metallic halides, and mixtures thereof. An electrolyte solute may be dissolved in the liquid cathode material in order to raise its conductivity. Specifically, the liquid cathode material may be thionyl chloride and the solute may be lithium tetrachloroaluminate. The anode of the cell is usually lithium or other highly electropositive metal. During discharge, the liquid cathode is electrochemically reduced on a cathode current collector. The cathode current collector may include a carbon layer supported on a conductive substrate.
A single cell may have one or more anodes alternately arranged with one or more cathode current collectors. Adjacent elements may be separated by porous insulating material such as glass paper. The cell is activated when the electrodes are wetted by the electrolyte solution.
Heat is generated within the cell by the exothermic electrochemical reaction and by ohmic losses during high current flow. Control of such heat is critical to safe operation. If the temperature near a lithium anode exceeds the melting point of the lithium, a rapid exothermic reaction of the lithium anode with other cell components can result. The quantity of reacting lithium needed for significant heat and cell rupture is quite small. In some cases, the source of heat can be quite localized resulting from a small metallic short between the electrodes.
One commonly used device to avoid excessive thermal excursions is a thermally activated switch which disconnects the external load to the cell in the event of excessive heat. However, this arrangement does not disconnect internal shorts.
It is known to place a temperature sensitive switch inside or in thermal contact with the cell case in order to sense excessive temperature and open the external circuit. While this arrangement has been generally used in the art, there are two disadvantages. First, the switch senses temperature at only one location and second, the conventional thermal switch cannot interrupt a short circuit created within the electrode stack where electric current passes between adjacent electrode plates.
It is an object of the invention to provide a material which is suitable for incorporation within elements of an electrochemical call and other articles for electrical disconnecting all or part of the article in response to excessive temperature.